The invention described herein relates to a class of new esters of L-carnitine and acyl L-carnitines and their use as cationic lipids suitable for favouring the intracellular delivery of pharmacologically active compounds, facilitating their transmembrane transport, or for promoting their interaction with specific cell membrane sites (receptors).
The invention described herein also relates to further known esters of L-carnitine and acyl L-carnitines, useful for the same purposes as the above-mentioned new compounds.
What is meant here by the term xe2x80x9cintracellular deliveryxe2x80x9d is cellular transfection with polynucleotides or plasmids of natural origin or modified, endowed with therapeutic activity (gene delivery) or the introduction of drugs or immunogenic peptides into the cells.
Many of the pharmacologically active substances, such as, for instance, polypeptides and proteins or drugs in general need to penetrate into the cells to exert their effects by influencing cell functions at subcellular or molecular level. For these molecules the cell membrane constitutes a selectively impermeable barrier. The cell membrane, in fact, performs a protective function, preventing the entry of potentially toxic substances, but also the passage of compounds with therapeutic activity. The complex composition of the cell membrane includes phospholipids, glycolipids and proteins; its function is influenced by cytoplasmatic components such as Ca++ and other ions, ATP, microfilaments, microtubules, enzymes and proteins that bind Ca++. The interaction between the structural and cytoplasmatic components of the cells and the response to external signals are responsible for the selectivity shown by and among the various different cell types. The barrier effect of the membranes can be overcome by combining substances in complexes with lipid formulations that reproduce the composition of naturally occurring membrane lipids. These lipids are capable of fusing with the membranes and of releasing the substances combined with them into the cells. The lipid complexes are capable not only of facilitating intracellular transfer by means of fusion with the membranes, but can also diminish the charge repulsion between the membrane and the molecule that has to penetrate into the cell. Amphipathic lipids, such as membrane phospholipids, form lipid vesicles or liposomes in the aqueous systems.
Liposomes are vesicles in which an aqueous volume is entirely enclosed by one or more membranes composed of lipid molecules, usually phospholipids. Phospholipids, which consist in a hydrophilic head and a pair of carbon chains (hydrophobic tail), are the main components of biological membranes. In aqueous solution the hydrophobic tails autoassociate to exclude water, while the hydrophilic heads interact with the medium, spontaneously forming populations of vesicles of varying diameters. The lipids are generally zwitterionic, neutral or anionic. These vesicles can be used as carriers of drugs, small molecules, proteins, nucleotides and plasmids.
Over recent years, the cationic liposomes, a class of positively charged vesicles prepared from synthetic lipids, have been extensively used for the transfer of genetic material into the cells. The negative charge of DNA can interact with the positive charges of the cationic lipids, forming a stable DNA-liposome complex. The simplicity and versatility of this technology have made liposomes an important vehicle for the delivery of genes for gene therapy in human subjects. Currently, most of the vectors used for gene therapy and approved by the NIH Recombinant Advisory Committee include viral and synthetic systems.
Viral infection involves a series of complex mechanisms in order to be able to attack a specific cell and carry the DNA into the nucleus. The rationale for the use of viral vectors for gene therapy is based on the possibility of replacing the viral genes with genes that code for a therapeutic function, without eliminating the ability of the viral particle to infect the cells. The limitations of viral therapy have to do with those viral elements that may be immunogenic, cytopathic and recombinogenic.
Great hopes are placed in the use of cationic lipids for gene therapy. These vectors possess great potential compared with those of biological origin, since they are much safer, less toxic and are also capable of incorporating genes of large size. As compared with biological-type vectors, however, they have a low intracellular gene transcription yield. It should be borne in mind, however, that the use of such transfection systems is in an initial stage of research. Cationic lipids play a very important role in the formation of the DNA-lipid complex, in cell-complex interaction, in fusion with the membrane, in DNA release inside the cell and in transcription.
There are important examples of in-vivo applications of cationic liposomes. The first clinical trial on gene therapy was conducted by introducing an expression vector containing the human liposome-complexed HLA-B7 gene for the treatment of melanoma. Another important application relates to the treatment of pulmonary cystic fibrosis by means of the administration via the pulmonary route or as a nasal spray of the liposome-complexed expression vector SV-40C-FTR. Other clinical trials involving the use of liposomes in gene therapy for cancer are currently in progress.
Four constituent elements are generally identified in the structure of cationic lipids: the positively charged cationic head, the spacer, the anchor lipid and the linker bond.
The cationic head is responsible for the interactions between cationic liposomes and DNA, between the DNA-liposome complex and the cell membrane and the other components of the cell. It consists of mono- or polycationic groups (depending on the number of charges) that can be variably substituted.
The spacer is the part of the molecule that separates the cationic head from the hydrophobic tail and is involved in ensuring optimal contact between the cationic head and the negative charges of the DNA phosphates.
The anchor lipid is the non-polar hydrocarbon part of the molecule and determines the physical properties of the double lipid layer, such as its rigidity and rate of exchange with membrane lipids.
What is meant by xe2x80x9clinker bondxe2x80x9d is the bond between the hydrocarbon chains and the rest of the molecule. This bond determines the chemical stability and biodegradability of the cationic lipids.
In recent years the use of liposomes has steadily increased in the cosmetics sector. The success of liposomes in this field is due to the fact that these compounds are very well tolerated by the skin. They are used both as vehicles for active ingredients and as compounds favouring the absorption of the latter.
The scientific and patent literature is rich in references to the preparation and use of liposomes; there are, however, very few references describing the use of carnitine derivatives useful for gene delivery, whereas for drug delivery no documents are available dealing with known techniques for the preparation of compounds remotely resembling those according to the invention described herein.
Patent application EP 0 279 887 describes the use of a derivative of carnitine, i.e. phosphatidyl carnitine, optionally in mixtures with other phospholipids and lipids (cholesterol, phosphatidyl choline, phosphatidyl serine), for the preparation of liposomes.
In the example provided regarding the preparation of liposomes, liposomes of phosphatidyl carnitine are produced which incorporate propranolol, a drug known to be active as an antihypertensive, anti-angina and antiarrhythmia agent. The carnitine derivative is used here on account of the pronounced myocardial tropism of carnitine. This tropism makes it possible to avoid the liposomes being metabolised by the liver, rather than reaching the desired target site.
The presence of phosphatidyl carnitine also makes it possible to administer the liposomes orally, since they are resistant to intestinal lipases.
In J. Med. Chem. 1998 Jun. 18;41(13):2207-15, a number of esters of L-carnitine useful for gene delivery are described, but they are not described or proposed as useful agents for drug delivery.
EP 559 625 B1 describes a number of esters of L-carnitine and acyl L-carnitines endowed with selective gastrointestinal tract muscle-relaxing activity.
In recent years molecular biologists have identified numerous defects at the chromosomal level that cause hereditary diseases in human subjects.
An important sector of modem medicine is concerned with the treatment of these hereditary genetic-based diseases by means of the use of gene therapy protocols.
As already mentioned, cationic liposomes are extensively used for the intracellular delivery of pharmacologically active compounds, facilitating transmembrane transport or promoting their interaction with specific cell membrane sites (receptors).
These vectors have great potential as compared to those of biological origin, since they are much safer, less toxic and are also capable of incorporating genes of large size. As compared to biological-type vectors, however, they have a low intracellular gene transcription yield.
Moreover, gene transfer mediated by conventional cationic lipids requires that plasmid DNA and cationic lipids be kept separate, and that their mixing be effected immediately before gene transfer.
Attempts to stabilise these polynucleotide complexes have so far failed to yield encouraging results; in fact, they remain stable only for a short period.